Sulawesi: Togian Islands & Gorontalo Bay

Age and character of the Togian Islands and their role in the evolution of the Gorontalo BayMichael Cottam and Robert Hall, completed project

This project aims to investigate when, how and why the Togian islands formed and their role in the development of Gorontalo Bay. Reports suggest subduction-like volcanic rocks of unknown age. We aim to investigate the age, petrology and geochemistry of these volcanic rocks, and obtain any evidence about the origins of these islands. Precise dating will also provide an indication of the amount of material they may have provided to the surrounding offshore basins. New seismic data from Gorontalo Bay show important extension and early rifting, at least 7 seconds of sediment at the western end, several depocentres, and strange conical features on the sea floor that may be deepwater carbonate build-ups on hydrocarbon seeps. There are thick sediments in depocentres on each side of the Togian islands.

Gorontalo Bay is one of the most enigmatic basins in East Indonesia, and there have been little data to help understand its origin or significance. It is a relatively deep basin with water depths between one and two kilometres, and Hamilton’s regional maps show up to five kilometres of sediment in its western depocentre. Oil seeps have been known from the southern edge of Gorontalo Bay, close to the East Sulawesi ophiolite, for many years.

To the north of Gorontalo Bay, the narrow north arm of Sulawesi has a long volcanic history and is interpreted to be built on Eocene oceanic crust. In contrast, at its western end there are two kilometre high mountains with young metamorphic ages where there is evidence of continental crust, Early Miocene extension and core complex formation. To the south, the east arm of Sulawesi is largely ophiolites, again forming high mountains, but in places there are garnet peridotites suggesting a sub-continental lithospheric origin.

In the centre of Gorontalo Bay, is the isolated active volcano of Una-Una which has an unusual K-rich chemistry and last erupted violently in 1983, Although Una-Una has been interpreted as the result of subduction of the Celebes Sea at the north Sulawesi trench, it is not a typical subduction volcano in composition or position (it is about 200 km above the Benioff zone), and if related to this subduction is unusual in being the only volcano.

Just southeast of Una-Una are the Togian islands, which form a broadly WSW-trending ridge that continues to the west as a submarine feature. They are known to be young volcanic islands, probably of Late Miocene age, although they are poorly dated. Tectonic plate reconstructions suggest the islands may represent a brief interval of volcanic activity formed by subduction as the east arm moved towards the north arm.